In the spread spectrum communication, as indicated in FIG. 9(a), the PN code, which is one of binary codes, is modulated with data and the carrier, which is modulated with the PN code thus modulated, is transmitted. In the figure, reference numeral 31 represents data; 32 is a modulator: 33 is a PN code generator; 34 is a carrier generator; 35 is a modulator; and 36 is an antenna. On the receiver side, as indicated in FIG. 9(b), the signals are received and correlated with a PN code serving as the reference. Self correlation waveform (in this specification, hereinbelow, called correlation spike waveform) having a relatively large amplitude appearing when the two codes are in accordance with each other or in the neighborhood thereof is treated to restore the data. In the figure, reference numeral 37 is an antenna; 38 is a correlator: 39 is a reference PN code generator; 40 is a data demodulator; and 41 represents data.
There is known a convolver as one of matched filters. A convolver is a functional element performing convolution integral and it serves as a matched filter performing a correlation operation, if the binary code serving as the reference (in this specification, hereinbelow, called reference code) is in the time inverted relation with respect to the received code.
There are known SAW convolvers as an example of convolvers. From the point of view of the construction there are convolvers, (1) in which an air gap is disposed between a piezo-electric body and a silicon layer, (2) in which a piezo-electric body and a silicon layer are formed in one body through an oxide layer, (3) which is composed only of a Piezo electric body, etc. All of these execute multiplication operation by interaction of the two signals, utilizing nonlinear characteristics and integrate the result of the interaction in an electrode called gate disposed in the interaction area.
FIG. 10 shows an example of the construction of the SAW convolver, in which reference numerals 42 and 43 are transducers; 44 is a piezo-electric body; 45 is an oxide film; 46 is a silicon substrate; and 47 is a gate electrode. The signal s(t) inputted through the transducer 42 propagates toward the right in the figure and the signal r(t) inputted through the transducer 43 propagates toward the left. The interaction between s(t) and r(t) is produced owing to the non-linear characteristics of the piezo-electric body--oxide film --silicon structure and the integral operation is executed and the result of the interaction is integrated by the gate electrode 47.
The signal c(t) outputted by the gate electrode 47 can be represented by the following equation; ##EQU1## where A is a constant; T represents the time necessary for acoustic wave to pass under the gate electrode (in this specification, hereinbelow, called in-gate delay time); x the distance measured in the direction of the propagation of s(t); and v the sound velocity.
In general the PN code has a determined period. In the waveform produced on the transmitter side it is often so constructed that there exists a certain relation between one period of the PN code and the length of one bit in the data. Here, in order to make explanation easier, as an example, it is supposed that the one period of the PN code and the length of one bit in the data are equal to each other.
On the other hand the relation between the in-gate delay time and the PN code can be also suitably selected. That is, the in-gate delay time can be either shorter than, equal to, or longer than the one period of the PN code. The in-gate delay time means an integral domain in the correlation operation. Taking the correlation characteristics of the PN code into account, it is desirable that the integral domain extends exactly over one period. Therefore, in this explanation, as an example, it is supposed that the in-gate delay time and the one period of the PN code are equal to each other.
The relations described above are shown in FIGS. 11(a), 11(b) and 11(c). FIGS. 11(a) and 11(b) represent the arrangement of the data and the PN code, respectively. In the above example the length of the one data bit and one period of the PN code are identical and both of them are equal to l. FIG. 11(c) is a schematical cross-sectional view of the convolver and the delay time within tho length L of the gate electrode is equal to l. The above description is an example for explaining this invention and the relations among the length of one data bit, one period of the PN code and the in-gate delay time can be arbitrarily selected.
For the real communication, since when transmitted signals are received is unknown on the receiver side, it waits for the reception of tho signals, while inputting the reference signal to one of the transducers. When a signal is received, it is supplied to the convolver through the other transducer. When the PN codes contained in the received signal and the reference signal, respectively, are in accordance to each other, the correlation spike waveform is obtained through the gate electrode of the convolver. However it is not known at all at which position they are in accordance. If the position where the two codes are in accordance were not correctly set, the data could not be restored correctly. For example, in the case where the two codes are in accordance in the form indicated in FIG. 12(a), on tho first half of the received code a data bit A takes place and on the second half another data bit B takes place. The figure shows the arrangement of the data bits, the received PN code and the reference PN code, the region indicated by L representing the interaction region under the gate electrode. The PN code A represents the time inverted code of the PN code A.
As explained above, some measures should be taken so that the received code and the reference code are in accordance finally at the position indicated in FIG. 12(b), wherever they are in accordance at first. Here the period from the moment, where the signal is received, to the moment, where the codes are in accordance with each other at the position indicated in FIG. 12(b), is called initial synchronization.
In the case where there exists a difference between the clock frequency of the received PN code and the clock frequency of the reference code, after the initial synchronization has been effected and the codes have been positioned, as indicated in FIG. 12(b), the position where they are in accordance with each other is shifted gradually from the arrangement indicated in FIG. 12(b). This shift can be represented, every time the beginning of the received PN code and that of the reference PN code encounter, by; ##EQU2## where f.sub.r represents the clock frequency of the reference PN code; f.sub.t the clock frequency of the received PN code; and N the number of chips constituting one period of the PN code.
That is, even if the initial synchronization is effected, when the clock frequencies of the codes are different the position where the two codes are in accordance with each other, is shifted gradually from the correct position and the demodulation to obtain the data becomes impossible. This means that a completely identical clock frequency should be used on the transmitter side and the receiver side. For a clock oscillator a quartz oscillator is usually used as the reference. Therefore this method has drawbacks that not only it is extremely difficult to fabricate plural quartz crystals having a completely same frequency, but also circumstances such as temperature, humidity, etc. should be controlled with an extremely high precision.
In order to remove the drawbacks described above, a method is proposed e.g. in Japanese Patent Application No. 59- 77789, by which the initial synchronization described above is effected by generating a pulse (hereinbelow called correlation pulse) by signal-processing the correlation spike described above and by making the patterns corresponding to one period of the two PN codes be in accordance with each other by initializing (resetting) the reference PN code while using the correlation pulse stated above.